Rocks and Soil 



305 



Irrigation. In localities where a natural water supply is 

 lacking, it becomes necessary to rely on irrigation, that is, on an 

 artificial supply of water. In the dry or arid regions of the west, 

 thousands of acres are made capable of production through irriga- 

 tion. Immense dams are constructed to hold the water. The 

 Roosevelt dam in the valley of Salt river is an illustration. This 

 dam forms a lake twenty-five miles long and two miles wide with 

 a depth in places of 225 feet. It furnishes water for over 200,000 



V. 8. Bureau of Reclamation. 



ROOSEVELT DAM 



A great irrigation project in Arizona. 



acres in the vicinity of the city of Phoenix, Arizona. It is one 

 of a large number of such dams built in various parts of the 

 west by the United States government. 



Dry Farming. In some semi-arid regions where the crops 

 ordinarily fail owing to insufficient moisture, a kind of cultiva- 

 tion known as dry farming is often employed. This method is 

 based on the fact that moisture does not evaporate rapidly from 



